I agree with Frost, you should use a DMM if you have one. If you dont have one, you should go buy one, $10 or something at radio shack. You will want one if you are into car audio. Anyways, you dont NEED to use one to find a good grounding spot. Make sure your ground is well sanded, it should be clean of dirt, grease, paint, basically you want a strong metal-->metal contact. Use a bolt to secure your ground if you have to drill your own hole, and use a ring terminal or something similar to contact the bolt.
If you dont want to get/use a DMM, dont worry about it, just sand, keep the wire pretty short, and make a strong connection. If you have alt whine/ or induced noise after you get everything set up, then you might want to find a better grounding spot.
By the way, ive never understood the whole "grounding to the battery" deal. I mean running a ground all the way back to the - terminal and then your battery itself gets grounded to the car chassis...
Im no expert though...so maybe i'm missing something.